1. Field of the Invention
The present invention relates to wireless communication system protocols. More particularly, the present invention relates to the field of encoding and decoding the length of a Medium Access Control (MAC) Protocol Data Unit (PDU) in a broadband wireless network.
2. Description of the Related Art
Broadband wireless networks are based on various communication standards, for example, the Institute of Electrical and Electronics Engineers (IEEE) 802.16e based Worldwide Interoperability for Microwave Access (WiMAX) standard, and its evolution to IEEE 802.16m, provides various types of services such as voice, packet data exchange, and the like. In such a network, user data and control information is exchanged between a Mobile Station (MS) and a Base Station (BS) by establishing a connection. The control information is generated by various protocols running at the MS and the BS. The data packets are generated by various applications at the MS and BS. Typically, wireless communication standards beyond 3G, for example 3GPP Long Term Evolution (LTE), IEEE 802.20, and IEEE 802.16e-2005, mobile WiMAX, and the like, use Protocol Data Units (PDUs) to carry the control and data information.
The IEEE 802.16m communication standard is associated with a protocol architecture that includes Physical (PHY) layer and Medium Access Control (MAC) layer specifications. The MAC layer comprises three sub-layers which include the specific Convergence Sub-layer (CS), the MAC Common Part Sub-layer (MAC CPS), and the security sub-layer. The CS provides transformation and/or mapping of external network data, received through the CS Service Access Point (SAP), into MAC Service Data Units (SDUs) received by the MAC CPS through the MAC SAP. This includes classifying external network SDUs and associating the external network SDUs to an appropriate MAC connection. It may also include other functions such as Payload Header Suppression (PHS).
The MAC CPS receives data from the various CSs, through the MAC SAP, classified to particular MAC connections. Data is transferred between the MAC CPS and the PHY layer via the PHY SAP. The MAC CPS also contains a separate security sub-layer that provides authentication, secure key exchange, and encryption. The MAC CPS forms an IEEE 802.16m based MAC PDU. The MAC PDU format includes a Generic MAC Header (GMH), an Extended Header (EH) group, and payload. Each connection payload includes one or more MAC SDUs or MAC SDU fragments received from the CS layer for the particular connection.
Further, the GMH format (based on the IEEE 802.16m standard) includes a Flow Identifier (Flow ID) field, an Extended Header (EH) field, and a Length field. A connection is identified by the Flow ID field. The EH field indicates whether an extended header group is present in the MAC PDU or not. Further, the Length field gives information about the length of the MAC PDU. The EH group includes a fixed portion that indicates the EH group length. The EH group also includes a variable portion following the fixed portion which consists of one or more EHs. The EH contains a type field and a body contents field. The type field indicates the type of EH and the body contents field indicates the type-dependent contents.
The MAC layer encodes multiple MAC PDUs (MPDUs) to form a MAC layer packet, which is then sent to a PHY layer for transmitting on the air. The size (in bytes) of the MAC layer packet formed by the MAC Layer is equal to the number of bytes requested by the PHY layer from the MAC layer. Thereafter, the multiple MAC PDUs that are encoded in the MAC layer packet are sent from the PHY layer of a transmitting communication device to a PHY layer of a receiving communication device.
The maximum size of the MAC PDU can be 2047 bytes because the size of the Length field in the GMH is ‘11’ bits. In a high data rate system, the MAC SDUs carried in the MAC PDU can be much larger than 2047 bytes. In order to carry such large MAC SDUs, the MAC SDUs need to be fragmented and multiple MAC PDUs, each carrying one fragment of the MAC SDU, are formed and concatenated together in one physical layer packet. This approach causes increased overhead because of the GMH overhead (2 bytes) and fragmentation/packing information overhead (2 bytes per MAC SDU fragment). For a MAC SDU, which is carried in ‘n’ MAC PDUs, the additional overhead of ‘4*(n−1)’ bytes is incurred. Accordingly, the MAC PDU needs to have a larger Length field to facilitate carrying of larger MAC SDUs in one MAC PDU.
Further, the process of carrying a large MAC SDU in the form of multiple MAC PDUs in a MAC layer packet also consumes more Automatic Repeat reQuest (ARQ) sequence numbers as an ARQ sequence number is assigned per MAC PDU. This leads to more frequent polling for a feedback request to avoid stalling of the transmitter ARQ window. Hence, the MAC PDU needs to have a larger Length field to facilitate carrying of larger MAC SDUs in one MPDU.
Also, in the current MAC PDU format, the MAC PDU can carry payload from multiple connections. The multiplexing approach reduces security overhead significantly. Hence, if ‘n’ connection payloads are multiplexed in one MAC PDU, then 12*(n−1) bytes of overhead are saved. The smaller size of the Length field does not allow multiplexing if the size of the multiplexed payload and the headers is greater than 2047 bytes. Therefore, multiple MAC PDUs are formed, one for each connection, and concatenated together in one MAC layer packet. This has an additional overhead of 2*(n−1)+12*(n−1) bytes for ‘n’ connection payloads. Thus, the multiplexed MAC PDU needs to have a larger Length field to facilitate multiplexing.
Hence there exists a need to efficiently encode and decode the length of the MAC PDU in the MAC layer packet.